Bolt tension indicating means

ABSTRACT

Improved flat annular washer for indicating tension on associated high tensile strength bolt. Washer bears against either bolt head or nut of given radius. Washer includes means for indicating stress on washer which visibly moves when stress on washer, which is related to tensile stress on bolt, reaches a preselected level.

CROSS-REFERENCES TO RELATED APPLICATONS

This application is a continuation-in-part of application Ser. No.825,558, filed on Aug. 18, 1977, now U.S. Pat. No. 4,164,164 thecontents of which are incorporated by reference herein in theirentirety. Application Ser. No. 825,558 is, in turn, acontinuation-in-part of application Ser. No. 704,768, filed July 13,1976 now abandoned, the contents of which are also incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to washers for high tensile strength nuts andbolts and particularly to washers having means for indicating when apredetermined tension has been realized on the bolt.

2. The Prior Art

The amount of tension in a high tensile strength bolt is a criticalparameter in steel fabrication. If the bolt is not tight enough, thenthe joint of which the bolt is part will not be as strong as designed.On the other hand, if the bolt is overstressed, a potential futurefailure is built into the joint.

Among the most common techniques for determining the tension in a boltis by the use of a wrench capable of delivering a certain torque, whichtorque is equated to a predetermined tension on the bolt. This commontechnique is recognized by everyone skilled in the art to be one that ishighly variable in result due to significant variations in frictionwhich could lead to requirements for high or low torque to twist a boltat any predetermined actual tension. Thus, the correlation is rough andthe indication can prove highly inaccurate. Such a method is describedand its disadvantages are discussed in an article which appeared in theMar. 6, 1975 issued of Machine Design at pages 78 to 82.

The deficiencies in a method of determining tension by measuring torquehave been recognized for a very long time and there have been numerousattempts to provide ways of yielding a more direct technique foraccomplishing this desirable end. Thus, in Seitz U.S. Pat. No. 2,394,812granted on Feb. 12, 1946, it is proposed to provide such an indicatingmeans by employing two nuts which are soldered together and thenapplying a wrench to one of the nuts, specifically the outer one, andtightening the two until the torque on the nut actually engaged by thewrench becomes so great relative to the other nut that the solder issheared. When the outer nut is thus severed from the inner, anindication of the bolt tension is given. The Seitz method has severalaspects that have limited commercial realization. First, it requires twonuts which add to the cost. Secondly, the Seitz lock nut still relies ontorque as an indication of bolt tension since it is the shear forceresuling from torquing that will cause the shearing of the solderbetween the two nuts. This being so, the Seitz method is still anindirect method with all of the resultng shortcomings of the standardtorque method.

In Lewis U.S. Pat. No. 3,174,386, a special washer is disclosed whichyields a direct indication of bolt tension. This is accomplished byproviding the washer with two flared out portions and a necked downcenter, which necked down center will actually collapse when the tensionin the bolt reaches a predetermined level. While Lewis thus gives adirect indication of bolt tension, rather than an indirect indicationdetermined from torque, the Lewis washer tends to be costly tomanufacture and the indicating portion is recessed between the bolt headand the adjacent plate and is not readily visible to the person applyingbolt tension. Moreover, it requires soft steel which is contrary toacceptable modern practice of forming washers for high strength boltsfrom heat treated hardened steel.

It has been proposed to incorporate in bolt heads or in washers avariety of materials which change their optical properties, such ascolor or plane of polarization, when subjected to certain minimumamounts of tension. For example, reference may be had to the Grikscheitet al U.S. Pat. Nos. 3,224,316 and 3,495,907. However, to the best ofapplicant's knowledge, such optical indicating means have not achievedany degree of commercial success, partly due to the inclusion of softoptical materials therein.

In Velthoven U.S. Pat. No. 3,783,734, it is proposed to use a speciallydesigned washer in conjuction with a specially designed nut or bolt headwhich cooperates to cause a frangible piece of the specially designedwasher to break off when a predetermined tension is achieved in thebolt. The specially designed nut or bolt head, however, requires areduction in the bearing surface provided by said nut or bolt head,thereby reducing the effective tension capable of being handled by saidnut or bolt.

In Crites U.S. Pat. No. 3,788,186, a stress indicating means isdescribed which provides for the incorporation of a frangible insert inthe head of a bolt or on a washer, which frangible insert will breakwhen a tension of certain proportions is achieved in the bolt. In theusual tightening of such a bolt by a socket wrench, the socket coversand therefore conceals the indication element during the course oftightening. Also, the Belleville type washer is not allowed by mostCodes for use with high tensile bolts.

There are a number of commercially available washers presently on themarket which do yield direct indications of bolt tension. Thus, forexample, there is a four-piece washer assembly termed "preloadindicating washer" that is manufactured by The Standard Pressed SteelCompany which is described in a publication entitled Fasteners Handbookat page 278. This same washer assembly is described in theaforementioned article in Machine Design at page 81. In addition,Illinois Tool Works, Inc. markets a so-called "square cone washer" whichis apparently described in U.S. Pat. No. 3,628,584. Additionally, Cooper& Turner, Inc. and Bethlehem Steel Company both market a load indicatorwasher each of which is apparently described in U.S. Pat. No. 3,187,621which is owned by Cooper & Turner, Inc. However, all three of thesecommercially available washers yield an indication that is not readilyvisible and often requires a guage or the like in order to determineprecisely what is being indicated by them. The "square cone washer" isessentially a Belleville type washer with the same objections cited forCrites. The Bethlehem washer does not provide full bearing between theundersurface of the nut or bolt and the top of the washer. In high heatapplications this could lead to a relaxation of bolt tension.

SUMMARY OF THE INVENTION

According to the invention, I have developed an improved flat annularwasher including means for visually indicating that the stresstransmitted to the washer by the bolt has reached a preselected level.In a preferred embodiment of the invention, the visual indicating meanscomprises a tab joined to the balance of the washer by a portion ofreduced cross section with at least a portion of the tab extending adistance from the center of the washer not less than the radius of thebolt head. The portion of reduced cross section is designed with apredeteremined stress failure which is reached when the stresstransmitted to the washer by the bolt reaches a preselected level. Thus,when the washer is stressed to the preselected level, the portion ofreduced cross section fails whereupon the tab moves relative to thebalance of the washer. This movement provides a visual indication thatthe preselected stress level has been reached.

In one embodiment, the tab and portion of reduced cross section aredefined by a slot in one surface of the washer along a chord thereof.However, this is not necessary, and other arrangements are possible. Forexample, in the presently preferred embodiment the tab is disposed in aperipheral recess in the washer and integrally joined thereto at theradially inward end of the recess by the portion of reduced crosssection.

In a variation of the preferred embodiment, the portion of reduced crosssection is rendered brittle as by case hardening or carburizing. As aresult, when the stress on the washer reaches a given level, the portionof reduced cross section fractures whereupon the tab separates from thebalance of the washer. By controlling the process utilized to render theportion of reduced cross section brittle, the stress at which saidportion fractures may be predetermined.

It has been found that in all these embodiments, it is preferable tostart with the tab at an angle with respect to the remainder of thewasher and have the bolt head overlie the portion of reduced crosssection, as this produces the most pronounced movement of the tab whenthe preselected stress level has been achieved. Also, it is preferablethat the surface of the washer having the slot which defines the portionof reduced cross section faced away from the bolt head. In thoseembodiments where the portion of reduced cross section does notfracture, the tab will then move to a position substantially coplanarwith the remainder of the washer when the preselected stress level hasbeen achieved.

In another embodiment of the invention, the washer is provided with agroove extending radially inward from its periphery, and the visualindicating means comprises a resilient element having a predeterminedbias equal to the preselected stress level.

Most preferably, the resilient element comprises a metal leaf spring. Aportion of the resilient element is complementary in shape to thegroove, whereby when said portion is disposed in the groove, one surfacethereof is coplanar with the grooved surface of the washer. Preferably,the portion of the spring disposed in the groove is not fixedly securedtherein whereby different leaf springs having different predeterminedbiases may be used with a single washer. However, if desired, the leafspring may be secured in the groove as by a suitable adhesive. Theremainder of the element extends outwardly from the periphery of thewasher.

When the washer is stressed to the preselected level, the predeterminedbias is overcome thereby effecting movement of the portion of theelement extending beyond the bolt head. This provides a visualindication that the preselected stress level has been reached. It ispresently contemplated that the resilient element will serve as a"monitor" of bolt tension and that it will return to its unstressedposition as the bolt tension decreases.

Further features and advantages of the improved washer according to thepresent invention will become more fully apparent from the followingdetailed description and annexed drawings of the preferred embodimentsthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a view partly in section and partly in elevation showing awasher embodying the present invention in connection with a nut and boltthat are employed to connect two plates together;

FIG. 2 is a bottom plan view of the washer of FIG. 1 showing theposition of the tab after the preselected tension has been achieved;

FIG. 3 is a sectional view of the washer of FIG. 2 but showing theposition of the tab before the preselected tension has been achieved;

FIG. 4 is a view similar to FIG. 3 showing the position of the tab afterthe preselected tension has been achieved;

FIG. 5 is a bottom plan view of another form of washer embodying thepresent invention showing the tab in the stressed position;

FIG. 6 is a sectional view of the washer of FIG. 5, but showing the tabin the unstressed position;

FIG. 7 is a view similar to FIG. 6, but showing the tab in the stressedposition;

FIG. 8 is a top plan view of a still further modification; and

FIG. 9 is a sectional view taken along the line 9--9 of FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings in detail and particularly to FIGS. 1-4thereof, a washer 10 which incorporates a tension indicating means 12 isshown in combination with a high tension bolt 14 and its cooperating nut16 which serve to join two structural elements, shown here by way ofillustation as two plates 18 and 20. The high tensile strength bolt 14is of conventional design, shown by way of illustration as having acastellated hexagonal head 22 and a threaded shank 24 which between themdefine a bearing surface 26. The nut 16 is of similar hexagonalcastellated configuration for the purposes of illustration and is shownto have approximately the same diameter as the bolt head 22. Thus, theannular bearing surface 28 defined by the nut 16 is substantially equalin area to the bearing surface 26. As is common, and as is preferred,the annular surface of the washer 10 is greater than the annular bearingsurfaces 26 and 28. In general, the aperture 30 in the washer 10 is ofabout the same diameter as the aligned apertures 32 and 34 in the plates18 and 20.

Save for the inclusion of the bolt tension indicating means 12 on thewasher 10, the arrangement heretofore described and illustrated in FIG.1 is wholly conventional and well known in the art. The materials fromwhich the various components are made may be any suitable materials,although it is abvious to the skilled art worker that the majorpractical application for this invention lies in steel constructionwherein the bolt, washer, nut and plates are all made of steel, the boltand nut being made of a high tensile strength steel.

As indicated, the present invention resides in the incorporation in thewasher 10 of a novel means for indicating the tension on the bolt 14. Itwill be clear from a perusal of FIG. 1 that as the tensile stress in thebolt 14 increases, the compressive stress on the washer 10 increasesconcomitantly. This being the case, if a tab 50 is joined to the balanceof the washer by a section 52 of diminished thickness as by a slot 54 inthe washer 10, the section 52 will be subjected to a downward force thatwill be dependent upon the tension in the bolt and not on torque or onany other parameter. Thus, by preselecting the thickness of section 52,washer 10 may be designed such that section 52 will fail at anypreselected tension. When section 52 fails, tab 50 will move, suchmovement providing a visual indication that the preselected tension hasbeen reached.

As presently preferred and best shown in FIG. 2, the slot 54 defines achord of the washer 10. Assuming, as shown in FIG. 1, that the washer isintended for disposition between the bolt head 22 and the plate 18, itwill be apparent that the required downward force on section 52 will beobtained only if the location of slot 54 is selected such that the bolthead overlies at least a portion of the secion 52. However, the bolthead should not fully cover the tab 50 so that movement of the tab willbe clearly visible. Thus, assuming, as is preferred, that the tab 50 isinitially in a deflected position (FIG. 3), as the bolt 14 is tightened,the downward force on the tab 50 will finally cause the tab 50 toflatten (FIG. 4) thereby giving a visual indication that a particulartension on the bolt has been reached. By proper selection of thedimensions of slot 54, the force required to flatten tab 50 may bepredetermined with a high degree of accuracy whereby flattening of thetab will correspond to a specific tension on the bolt.

In FIG. 2, it will be noted that the thinned section 52 is defined bothby the slot 54 and the lateral notches 56. Presently, this configurationis preferred. However, once this description is known, various otherarrangements for defining a thinned portion will suggest themselves tothose skilled in the art. For example, the transverse slot 54 could beeliminated and the thinned portion defined solely by the lateral notches56. Alternatively, the notches 56 could be eliminated and the slot 54retained. Suffice it to say it is desirable to select an arrangementwhich will result in maximum deflection for any given load.Considerations relevant to selecting such arrangements may be found, forexample, in Peterson's Stress Concentration Design Factors.

While it is preferred that tab 50 start in the deflected position, thisis not mandatory and the tab 50 could also start in the flattenedposition with deflection indicating that the predetermined bolt tensionhas been reached. If this modification is employed, it will be apparentthat the bolt head 22 must terminate substantially at the slot 54 inorder that tighetning of the bolt 14 will result in upward flexing oftab 50. Desirably, the bolt head should extend as close to the slot 54as possible inasmuch as it has been found that the washer will notdistribute much loading past the point of concentrated load.

Still another possibility is to use the embodiments of FIGS. 2-4 withthe slot 54 confronting the bearing surface of the bolt head 22. If thisvariation is employed it will again be apparent that in order toaccommodate upward flexing of tab 50, bolt head 22 should not overlieslot 54. However, to insure sufficient compressive loading in the region52, bolt head 22 should extend as close as possible to the slot.

Of these three variations of the embodiment of FIGS. 2-4, the first ispreferred inasmuch as it results in the greatest movement of tab 50 forany given compressive force and thus provides the most readily visibleindication that the predetermined bolt tension has been reached.

The nature of the material of the tab 50 may be any suitable material,preferably brightly colored for easy visibility and possibly coloredcoded as to stress. In the embodiments of FIGS. 2-4, the tab 50 ispreferably formed integrally with the washer 10 and thus formed of thesame material as the washer. While a single tab 50 is shown, it willalso be apparent that several such tabs may be incorporated in a singlewasher, with the possibility that each will indicate a differentpredetermined bolt tension. Furthermore, while the washer 10 is shownbearing against the bolt head 22, it will be apparent from the teachingsherein that it may be employed equally effectively in bearing relationwith the nut 16 as well. Thus, in all previous and subsequentdescription, this alternative use will be recognized as existing andpracticable, as will the alternative of incorporating visual indicatingmeans 12 in an integral nut and washer assembly.

Referring now to FIGS. 5-7, a modified washer 10 similar to that ofFIGS. 2-4 is shown. The washer 10 of FIGS. 5-7 has a recess 60 and thetab 62 extends radially outward from the recess and is joined thereto bya portion of reduced cross section 64 defined by a slot 66 and lateralnotches 68. In the washer 10 of FIGS. 5-7 the diameter of the washer ispreferably selected such that the bolt head 22 overlies the thin portion64 but does not fully cover the tab 62. The operation of the washer inFIGS. 5-7 is the same as that of FIGS. 2-4 with the tab 62 being shownin FIG. 6 in its initial, deflected position and in FIG. 7 in its final,flattened position, which it assumes when the predetermined tension onthe bolt 14 has been reached. It will again be apparent that theflattening of the tab 62 will be readily visible to the worker applyingtension to the bolt 14. Presently, the embodiment of FIGS. 5-7 is mostpreferred of all the embodiments described herein.

In the embodiment of FIGS. 5-7, it may be desirable to form the tab 62separately from the balance of the washer 10. If so, the portion 64 maybe joined to the balance of the washer by, for example, welding or asuitable adhesive. If this is done, the joint should be selected towithstand greater compressive loading than that required to move tab 62,lest the joint fails before the tab deflects.

It should be understood that the variations discussed above inconnection with the embodiments of FIGS. 2-4 are equally applicable tothe embodiments of FIGS. 5-7. Still another possibility equallyapplicable to both embodiments is heat treating the thin portion toeffect fracturing rather than deflection. For example, washers aretypically hardened as by case hardening or carburizing, in which thesurface of the washer becomes inpregnated with carbon. If theseprocesses are applied to the washer after the portion of reduced crosssection is formed, the reduced portion will be almost completely carbonimpregnated. As a result, the reduced portion will be quite brittle,subjecting it to fracture under sufficient load. By varying the degreeof carbon impregnation, the load under which the reduced portion willfracture may likewise be varied whereby fracturing will provide a visualindication that a predetermined bolt tension has been reached.

Referring now to FIGS. 8 and 9, another embodiment of a washer 10including visual indicating means 12 according to the present inventionis shown. The washer of FIGS. 8-9 includes a radial slot or groove 70.Visual indicating means 12 comprises a strip of resilient material 72which serves as a spring. The spring 72 is preferably at rest in theflattened position (solid lines in FIG. 9).

When the surface of the washer 10 including groove 70 and spring 72 isdisposed in confronting relation with the bolt head and the bolt is thentightened, the spring 72 will be subjected to a force dependent on thetension of the bolt. When this force is sufficient, the spring 72 willdeflect at its juncture with the bolt head 22 (dotted lines in FIG. 9)thereby giving a visual indication that a particular bolt tension hasbeen reached. By preselecting the spring force, the force required todeflect the spring 72 may be predetermined with a high degree ofaccuracy whereby deflection of the spring will correspond to a specificbolt tension.

The embodiment of FIGS. 8-9 is intended to serve as a reusable "monitor"of bolt tension. That is, once the bolt is loosened, it is contemplatedthat the spring 72 will return to its initial flat position (solid linesin FIG. 9). Thus, the embodiment of FIGS. 8-9 provides a continuousindication as to whether or not the desired bolt tension is present.

The spring 72 may be comprised of any suitable resilient material. Forexample, metals and their alloys may be used and are preferred. However,the spring 72 may be comprised of suitably hard plastics as well.Desirably, the spring is not secured to the washer and is instead simplyslipped into the groove 70 just before the bolt is tightened. Thispermits different springs having different spring forces to be used inconnection with a single washer. Of course, if desired, the end of thespring 72 in the groove 70 could be secured therein as by a friction fitor a suitable adhesive.

Again, it will be appreciated that most of the modifications and changessuggested above in connection with FIGS. 1-7 are equally applicable tothe embodiment of FIGS. 8-9.

As a further alternative for use in connection with the embodiment ofFIGS. 8-9, the spring 72 may be replaced by a similarly dimensionedstrip comprised, for example, of soft plastic. As the bolt head istightened, the plastic strip will be subjected to a force dependent onthe tension of the bolt. Depending upon the materials selected for thestrip, the force applied to the strip by the bolt will eventually crushthe strip substantially at its juncture with the bolt head 22. Theresult will be an upward deflection of the portion of the stripextending beyond the bolt head thereby giving a visual indication that aparticular bolt tension has been achieved. The force required to crushthe strip will, of course, depend upon the material from which the stripis made. By preselecting the material, the force required to deflect thestrip may be predetermined with a high degree of accuracy wherebydeflection will correspond to a specific bolt tension. Of course, whendeflection is caused by crushing, the strip will not return to itsinitial or flat position upon loosening of the bolt. Accordingly, thismodification is not suitable for use as a monitor of bolt tension.

Any one of the washers 10 described hereinbefore may be employed inassociation with a pneumatic or electric torque wrench and theindicating means 12 may be operatively connected to a suitable signalproducing mechanism to provide a signal input for the wrench to stopwhen the means 12 indicates that the prescribed tension has beenachieved. Clearly, a simple electric or pneumatic servo system can beemployed in this manner once it is recognized that the washer 10 isavailable to the skilled art worker for such use.

When the various embodiments described hereinabove are used inconnection with a socket-type wrench, it may be desirable to remove, asby milling, the portion of the upper surface of the visual indicatingmeans 12 adjacent the bolt head 22 as this reduces the possibility ofinterference between the wrench and the indicating means.

Irrespective of the form of the invention employed herein, it will beapparent to anyone of ordinary skill that the incorporation of my stressindicating means in any of its forms will not render the washer (or nutor bolt) non-compliant with current codes and, thus, it will not benecessary to effect amendments to such codes to bring aboutcommercialization of the invention hereinbefore described in all of itsforms.

While I have herein shown and described the preferred form of thepresent invention and have suggested several modifications therein,other changes and modifications may be made therein within the scope ofthe appended claims without departing from the spirit and scope of thisinvention.

What is claimed is:
 1. In a flat annular washer of the type suitable foruse in cooperation with a nut or a bolt head having a given radius, theimprovement which comprises means for indicating that the stress on saidwasher has reached a preselected level, said indicating means comprisingan element joined to said washer by a portion of reduced cross section,at least a portion of said element extending a distance from the centerof said washer not less than the radius of said nut or bolt head, saidportion of reduced cross section having a predetermined stress failurewhich is reached when said washer achieves said preselected stresslevel, whereby when said washer is stressed to said preselected level,said portion of reduced cross section fails thereby effecting movementof said element relative to said washer.
 2. The washer of claim 1,wherein said element is a tab.
 3. The washer of claim 2, wherein saidtab is defined by a slot in a surface of said washer along a chordthereof.
 4. The washer of claim 2, wherein said washer has a peripheralrecess, and wherein said tab is disposed in said recess and integrallyjoined to said washer at the radially inward end of said recess by saidportion of reduced cross section.
 5. The washer of claim 3 or 4, whereinsaid tab is angled with respect to said washer toward said nut or bolthead, said nut or bolt head overlies at least part of said portion ofreduced cross section, and said tab moves into substantially coplanarrelation with said washer when said portion of reduced cross sectionfails.
 6. The washer of claim 3 or 4, wherein said tab is coplanar withrespect to said washer, said nut or bolt head terminates substantiallyat said portion of reduced cross section, and said tab moves out ofcoplanar relation with said washer when said portion of reduced crosssection fails.
 7. The washer of claim 2, wherein said tab separates fromsaid washer along said portion of reduced cross section when said washeris stressed to said preselected level.